Not Applicable
Not Applicable
1. Field of the Invention
The present invention is in the field of devices used in placing a cement plug in an open hole well bore, or in a casing in a well bore.
2. Background Art
It is sometimes desirable to segregate an uphole portion of a well bore from an adjacent downhole portion. Typically, the well bore or casing is full of drilling fluid having a density calculated to balance the formation pressure. Cement slurry is pumped to the location where the operator desires to segregate the well bore. A cement plug is deposited at this location and left to hydrate or harden. Usually, the density of the cement slurry is greater than the density of the drilling fluid. So, the cement tends to drift or migrate downwardly in the well bore, with the result that the cement plug is formed at a deeper location than desired. Further, because of this migration of the cement slurry into the drilling fluid, drilling fluid can flow upwardly through the cement slurry, and the cement plug formed may not completely extend across the diameter of the well bore. Still further, mixing of the drilling fluid with the cement may contaminate the cement plug and reduce its strength. Therefore, when a well bore or casing is to be plugged with cement, it is desirable to be able to limit or eliminate the migration or contamination of the cement slurry.
One known remedy for the cement migration or contamination problem is to place a barrier at the desired location for the cement plug, and then to place the cement slurry on top of the barrier. The barrier might be a mechanical apparatus or a jelled plug. Such mechanical barrier devices usually suffer from the disadvantage that the exact configuration and dimensions of the hole must be known, so they must be installed in a cased hole. Jelled barriers may take a long time to set up before the cement slurry can be placed. At least one mechanical barrier is also known, which consists of two sets of thin strips of metal, one angled uphole and one angled downhole, which flex and press outwardly against the walls of the bore hole or casing. This device relies upon additional uphole and downhole pumpable plugs to pump the barrier into position, and it is often difficult to determine when the barrier has been placed at the desired location. Further, because the device is centered only by the two sets of flexed metal strips, it may not be accurately centered in the hole. Finally, because of the downhole set of flexible strips, this device can become snagged in some types of work string.
The present invention comprises a pumpable barrier which has a plurality of flexible elongated flat strips, and an annular seal for sealing against the bore hole wall. The annular seal, which can be mounted at the uphole or downhole end of the barrier, provides a fluid barrier against which hydraulic pressure can be applied, to pump the barrier downhole, through a work string, to the desired location. This eliminates the need for pumpable plugs to be used to place the barrier downhole. The plurality of elongated flat strips are attached, at one end, to the body of the barrier, with their free ends being oriented toward the trailing end of the barrier. This allows the barrier to be pumped through any type of work string without snagging. The flat strips can be constricted closely to the body of the barrier, to allow pumping of the barrier through the work string, or they can be allowed to flex outwardly, to contact and press against the wall of the well bore. Once the barrier has been pumped downhole to the desired location, it exits the end of the work string, and the elongated strips are released to flex outwardly. Cement slurry placed above the barrier will not migrate through the barrier, because the spacing between the flat strips is too small to allow the passage of the viscous slurry. This allows the cement plug to completely set up, at the desired location, completely covering the cross section of the casing or well bore.
If desired, the barrier can be landed in a landing sub at the downhole end of the work string, then released from the landing sub by the shearing of a shearable device, such as a shear pin. Landing of the barrier in the landing sub results in an increase in pressure, signaling the operator that the barrier has reached the desired location. Release of the barrier from the landing sub results in a decrease in pressure, signaling the operator that the barrier has been put in place.
The novel features of this invention, as well as the invention itself, will be best understood from the attached drawings, taken along with the following description, in which similar reference characters refer to similar parts, and in which: